1. Field of the Invention
The present invention relates to light emitting diodes (LEDs) and to a technique for coating phosphors on an LED chip using electrophoresis.
2. Discussion of the Background
An LED packaging process may include forming a highly efficient optical system using LED chips, phosphors, and encapsulation members. In a typical LED package design, the encapsulation member of such an optical system may be formed using an encapsulation material such as epoxy resin or silicone gel.
However, the typical LED package design may include some optical incompatibilities. As a result, Fresnel reflection and total internal reflection loss may occur in the LED package due to a difference in refractive indices between the LED chips, the phosphors, and the encapsulation material.
The typical LED package design may also provide non-uniform light radiation from an LED. As a result, it may be more difficult to obtain a uniform radiation angle of light beams due to volumetric distribution of the phosphors in composites having different thicknesses. Further, white light may have a blue-shifted region (that is, light biased toward a blue color) and a yellow-shifted region (that is, light biased toward a yellow color).
The typical LED package design may also provide a local density variance, which may make it difficult to obtain a permanent optical coordinate.
The typical LED package design may cause the phosphors to be wasted. That is, the phosphors may be wasted due to the necessary use of volumetric composite coating.
FIG. 1 is a diagram showing a principle of electrophoresis for use in coating phosphors on an LED chip. Referring to FIG. 1, charged phosphors are deposited on an LED substrate that acts as part of a cathode electrode in relation to an anode electrode. In other words, electrophoresis enables the phosphors to be directly coated at high density to the LED substrate or an LED chip without using a conventional epoxy or silicone matrix and changing an effective refractive index of the phosphors. When a phosphor has a high refractive index, the phosphor also provides high optical output after being deposited. The electrophoresis approach may be useful for high output power white LEDs that use a large size LED (for example, 1×1 mm2 or more).
Examples of a method of depositing phosphors by electrophoresis are disclosed in U.S. Pat. No. 6,576,488, U.S. Pat. No. 6,642,652, U.S. Pat. No. 6,686,581, and U.S. Patent Publication No. 2007/0045761 assigned to Lumlieds Lighting Co. Further, U.S. Pat. No. 6,864,110 and U.S. Pat. No. 6,924,233 assigned to Agilent Technology Inc., and JP 2006-21049 assigned to Nichia Kagaku Kogoy K. K. disclose a method of depositing charged phosphors on a conductive surface of an LED chip using electrophoresis to form a uniform phosphor layer that produces uniform white light without a colored ring phenomenon.
Fine phosphor particles may be used to prepare a homogeneous suspension for electrophoresis deposition. The phosphor particles generally have a size of 2 to 5 μm in the suspension. An apparatus for electrophoresis deposition includes an agitator that maintains a mixed state of the suspension in a bath. The suspension may be sufficiently mixed through agitation for 24 hours, for example.
When the phosphor layer is composed of the fine phosphor particles it may provide relatively inappropriate wavelength conversion efficiency to the phosphor particles. Reducing the size of the phosphor particles may lead not only to significant reduction in settlement of the phosphor particles, but also to reduction in wavelength conversion efficiency of the phosphors. Typically, when the size of silicate phosphors is reduced by half, from 20 μm down to 10 μm, the wavelength conversion efficiency may be reduced by about 10 to 20%. Generally, phosphor particles having a diameter of 15 μm maintain higher radiation efficiency.
Accordingly, large-size phosphor particles may be advantageously used to increase phosphor coating efficiency on one hand, but on the other hand may form an incomplete phosphor coating layer due to a low density of the large-size phosphor particles once deposited. One solution is to increase the thickness of the phosphor coating layer to increase the density of phosphors. But increasing the thickness of the phosphor coating layer may cause excessive consumption of phosphors, high internal absorption, and scattering of light by large-size phosphor particles, and the like.